Switching power supply into the era of high efficiency power conversion

The continuous miniaturization of electronic devices, especially computers, requires that the size of the power supply be miniaturized. Therefore, the Switching Power Supply starts to replace the linear regulated power supply featuring a bulky power frequency transformer, and the power supply efficiency is significantly improved. The reduction in the volume of the power supply means that the heat-dissipation capability is degraded, and thus the power consumption of the power supply is required to be smaller, that is, the efficiency must be increased under the premise of unchanged output power.

High-Efficiency Power Conversion: The Goal of Switching Power Supply Design

The power dissipation of the same volume of power supply is basically the same. Therefore, to obtain a larger output power, the efficiency must be improved. At the same time, high power supply efficiency can effectively reduce the stress of the power semiconductor device, which is advantageous to improve its reliability.

Switching power supply losses are mainly: passive component loss and active component loss

Switching losses have been puzzling the designer of switching power supply. Because power semiconductor devices have current and voltage at the same time in the switching process, switching losses are inevitable. If switching power supply and output rectifier diodes in switching power supply can achieve zero voltage switching Or zero current switch, its efficiency can be significantly improved.

The switching loss caused by the switching process will roughly account for 5% to 10% of the total input power. Significantly reducing or eliminating this loss will increase the switching power supply efficiency by 5% to 10%. The most effective method is soft switching or zero voltage switching or zero current switching.

Among the many soft-switching solutions, a high-powered full-bridge converter is more practical, and a phase-shifting and zero-voltage switching control method is usually adopted. This control method requires a freewheeling inductor on the primary side to ensure that the switching tube is Turned on at zero voltage, this additional inductor will heat up (although much smaller than the RC snubber circuit) due to the larger rms current flowing, and is not used in low voltage power conversion.

The passive lossless snubber circuit is characterized by not destroying the conventional PWM control mode, and the design/debugging is simple. Despite this, passive lossless snubber circuits and quasi-resonant/zero voltage switch modes of operation also have some disadvantages, such as the ability to turn off soft switching only and in flyback converters it is not well suited for large load range variations. Active clamping in soft switching is an effective way to increase the efficiency of single-transistor flyback converters. The original patent restrictions have now become invalid and can be applied universally.

Advances in Power Semiconductor Devices: The Fundament of High Efficiency Power Conversion

Advances in power semiconductor devices, and especially in PowerMOSFETs, have led to a series of advances in power conversion: the extremely fast switching speed of PowerMOSFETs has increased the switching frequency of switching power supplies from 20kHz to 100kHz above bipolar transistors, effectively reducing The volume of passive energy storage elements (inductors, capacitors). Low-voltage PowerMOSFETs make it possible to synchronously rectify low voltages. The device's turn-on voltage is reduced from about 0.5V for Schottky diodes to 0.1V or even lower for synchronous rectifiers, which increases the efficiency of low-voltage rectifiers by at least 10%. The high voltage PowerMOSFET's turn-on voltage drop and switching characteristics improve the primary efficiency of the switching power supply. The reduction of power consumption of power semiconductor devices also reduces the size of the heat sink and the overall machine.

There is an unwritten view in the power supply industry: unregulated is more efficient than regulated, non-isolating is more efficient than isolating, and a narrow range of input voltage is more efficient than a wide range of inputs. Vicor's 48V input power module achieves 97% efficiency. AC input switching power supply requires power factor correction. Since power factor correction already has a voltage regulation function, power factor correction and non-regulating isolation can be used in applications where the output ripple is less demanding (such as an output connected to a battery or a super capacitor). Inverter circuit topology, foreign products in 1986, efficiency reached 93%.

In a power module with a DC48V input voltage, a module with an efficiency of over 93% almost exclusively adopts a pre-regulator and a post-regulatory isolation scheme, and outputs a first-stage output capacitor and a second-stage output inductor. Cancellation simplifies the circuit structure.

Many domestic switching power supplies have relatively insufficient attention to structural design in their design. Occasionally, there are uneven temperature rises in various parts of the power supply, and some places are overheated. In some places, there is almost no temperature rise, and even large losses occur on the PCB. . A good switching power supply should be that the heat generating components are evenly distributed on the PCB, and the temperature rise of the heating components is basically the same, and the PCB should have as little loss as possible, which is particularly important in the design of the Adapter of the module power supply and plastic housing.

At the same time as the efficiency increases: The electromagnetic interference of the power supply is reduced

In the various losses of the switching power supply, the losses caused by electromagnetic interference will not be ignored after the power supply efficiency reaches a certain level. On the one hand electromagnetic interference itself consumes energy. In particular, the improvement of power supply efficiency often requires soft-switching technology or zero-voltage switching or zero-current switching technology (regardless of whether it is specifically set or inherent in the circuit itself). Application of these technologies slows the voltage and current during the switching process. The rate of change or elimination of the switching process, the electromagnetic interference becomes very small, and there is no need for a circuit that needs to be specifically set to suppress electromagnetic interference in the conventional switching power supply circuit (this circuit is subject to loss).

Switching Power Supply Entry: Efficient Power Conversion Era

Carefully analyzed, the high-efficiency power conversion seems to be very simple, and even some circuit topologies were introduced more than 20 years ago (such as a two-level conversion topology, as described in AN19 of the Application Note of the UNITRODE 82/83 data sheet. This power conversion topology is also used in the TEK2235 oscilloscope, but it is subject to the current state of the art, especially the limitations of people's understanding (it is always considered that the efficiency of two-level conversion is lower than that of single-stage, and in fact two-level conversion can realize the fact The inherent zero-voltage switching, single-stage conversion requires special additional circuitry and control methods, and has not been recognized and applied. Improvements in device performance and people's awareness have made the two-stage conversion one of the main ways of high-efficiency power conversion.

Conclusion

Today's switching power supply design engineers and manufacturers, advanced power semiconductor devices can be easily obtained, advanced circuit topology and control methods have begun to apply, what they are left is to find ways to improve their technical level, while creating better Application opportunities and market share.